Another idea I have had is to develop a mod in game that will fire an event if a player completes a special accomplishment this trigger can be picked up by the stats and bonus points could be awarded to the player. This could cause players to focus on special achievements as a way to improve score and thus will switch up game play a bit more. Examples include:
Turn 30 players into a zombie in 10min
Kill 100 zombies in one map
Stay alive as a human for 5 rounds
ect ect...